# Entity/Relationship Modelling

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Entity/Relationship Modelling

Topics To Be Covered In This Lecture • Entity/Relationship models
• Entities and Attributes • Relationships • Attributes • E/R Diagrams

Component of ERD Entity = An entity may be an object with physical existence (for example, a particular person, car, house or employee) or it may be a conceptual existence (for example a particular company, a job, a university course). Attribute = property of an entity set. – Generally, all entities in a set have the same properties. – Attributes are simple values, e.g. integers or character strings. For example: An EMPLOYEE entity may be described by the employee age, address, salary, and a job.

Symbols Of Entity Relationship Model

E/R Diagrams • In an entity‐relationship relationship diagram, each entity set is represented by a rectangle. Each attribute of an entity set is represented by an oval, with a line to the rectangle representing its entity set. Ideas E/R Design Relational Schema Relational DBMS

Relationship: A relationship among the two or more entities represent an association among two or more entities. For example work-on relationship between an employer and a project. One-To-Many Relationships A one-to-many relationship is the most common type of relationship. In this type of relationship, a row in table A can have many matching rows in table B, but a row in table B can have only one matching row in table A.

Many-To-Many Relationships
In a many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa. You create such a relationship by defining a third table, called a junction table, whose primary key consists of the foreign keys from both table A and table B.  One-To-One Relationships In a one-to-one relationship, a row in table A can have no more than one matching row in table B, and vice versa. A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints.

Derived Attribute: The attribute which actually doesn’t exist but is driven on the basis of pre-exist data, e.g. DOB is entered instead of age. Single Valued Attribute: Most attribute have a single value for a particular entity; such attribute are called single-valued attribute. Multivalued Attribute: Multivalued Attribute consist of more than one value. For example one person may not have a college degree, another person may have two college degrees, and a third person may have three college degree.

Tuples: Row are known as tuple Cardinality: No of rows or tuples is known as cardinality. Degree: No of attribute are known as degree.

Conceptual design • Build a model independent of the choice of DBMS Logical design Create the database in a given DBMS • Physical design • How the database is stored in hardware

Example • In a University database we might have entities for Students, Modules and Lecturers. Students might have attributes such as their ID, Name, and Course, and could have relationships with Modules (enrolment) and Lecturers (tutor/tutee)

Entities Entities represent objects or things of interest
Physical things like students, lecturers, employees, products. More abstract things like modules, orders, courses, projects. In an E/R Diagram, an entity is usually drawn as a rectangular box. • The box is labelled with the name of the class of objects represented by that entity

Difference B/W Entities and Instances
Entities have a general type or class, such as Lecturer or Module Instances of that particular type, such as Steve Mills, Robert James are instances of Lecturer. Attributes (such as name, address)

Overview of Database Design Process
Two main activities: Database design Applications design Focus in this chapter on database design To design the conceptual schema for a database application Applications design focuses on the programs and interfaces that access the database Generally considered part of software engineering

Overview of Database Design Process

Example COMPANY Database
We need to create a database schema design based on the following (simplified) requirements of the COMPANY Database: The company is organized into DEPARTMENTs. Each department has a name, number and an employee who manages the department. We keep track of the start date of the department manager. A department may have several locations. Each department controls a number of PROJECTs. Each project has a unique name, unique number and is located at a single location.

Example COMPANY Database (Contd.)
We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate. Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee. Each employee may have a number of DEPENDENTs. For each dependent, we keep track of their name, sex, birthdate, and relationship to the employee.

ER Model Concepts Entities and Attributes
Entities are specific objects or things in the mini-world that are represented in the database. For example the EMPLOYEE John Smith, the Research DEPARTMENT, the ProductX PROJECT Attributes are properties used to describe an entity. For example an EMPLOYEE entity may have the attributes Name, SSN, Address, Sex, BirthDate A specific entity will have a value for each of its attributes. For example a specific employee entity may have Name='John Smith', SSN=' ', Address ='731, Fondren, Houston, TX', Sex='M', BirthDate='09-JAN-55‘ Each attribute has a value set (or data type) associated with it – e.g. integer, string, subrange, enumerated type, …

Types of Attributes (1) Simple Composite Multi-valued
Each entity has a single atomic value for the attribute. For example, SSN or Sex. Composite The attribute may be composed of several components. For example: Address(Apt#, House#, Street, City, State, ZipCode, Country), or Name(FirstName, MiddleName, LastName). Composition may form a hierarchy where some components are themselves composite. Multi-valued An entity may have multiple values for that attribute. For example, Color of a CAR or PreviousDegrees of a STUDENT. Denoted as {Color} or {PreviousDegrees}.

Types of Attributes (2) In general, composite and multi-valued attributes may be nested arbitrarily to any number of levels, although this is rare. For example, PreviousDegrees of a STUDENT is a composite multi-valued attribute denoted by {PreviousDegrees (College, Year, Degree, Field)} Multiple PreviousDegrees values can exist Each has four subcomponent attributes: College, Year, Degree, Field

Example of a composite attribute

Entity Types and Key Attributes (1)
Entities with the same basic attributes are grouped or typed into an entity type. For example, the entity type EMPLOYEE and PROJECT. An attribute of an entity type for which each entity must have a unique value is called a key attribute of the entity type. For example, SSN of EMPLOYEE.

Entity Types and Key Attributes (2)
A key attribute may be composite. VehicleTagNumber is a key of the CAR entity type with components (Number, State). An entity type may have more than one key. The CAR entity type may have two keys: VehicleIdentificationNumber (popularly called VIN) VehicleTagNumber (Number, State), aka license plate number. Each key is underlined

Displaying an Entity type
In ER diagrams, an entity type is displayed in a rectangular box Attributes are displayed in ovals Each attribute is connected to its entity type Components of a composite attribute are connected to the oval representing the composite attribute Each key attribute is underlined Multivalued attributes displayed in double ovals See CAR example on next slide

Entity Type CAR with two keys and a corresponding Entity Set

Entity Set Each entity type will have a collection of entities stored in the database Called the entity set Previous slide shows three CAR entity instances in the entity set for CAR Same name (CAR) used to refer to both the entity type and the entity set Entity set is the current state of the entities of that type that are stored in the database

Initial Design of Entity Types for the COMPANY Database Schema
Based on the requirements, we can identify four initial entity types in the COMPANY database: DEPARTMENT PROJECT EMPLOYEE DEPENDENT Their initial design is shown on the following slide The initial attributes shown are derived from the requirements description

Initial Design of Entity Types: EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT

Refining the initial design by introducing relationships
The initial design is typically not complete Some aspects in the requirements will be represented as relationships ER model has three main concepts: Entities (and their entity types and entity sets) Attributes (simple, composite, multivalued) Relationships (and their relationship types and relationship sets) We introduce relationship concepts next

Relationships and Relationship Types (1)
A relationship relates two or more distinct entities with a specific meaning. For example, EMPLOYEE John Smith works on the ProductX PROJECT, or EMPLOYEE Franklin Wong manages the Research DEPARTMENT. Relationships of the same type are grouped or typed into a relationship type. For example, the WORKS_ON relationship type in which EMPLOYEEs and PROJECTs participate, or the MANAGES relationship type in which EMPLOYEEs and DEPARTMENTs participate. The degree of a relationship type is the number of participating entity types. Both MANAGES and WORKS_ON are binary relationships.

Relationship instances
In the mini-world represented by figure below employees e1, e2, e3 and e6 work for department d1, e2 and e4 works for d2 and e5 and e7 work for d3.

Relationship type vs. relationship set (1)
Is the schema description of a relationship Identifies the relationship name and the participating entity types Also identifies certain relationship constraints Relationship Set: The current set of relationship instances represented in the database The current state of a relationship type

Relationship types and instances
17

Relationship type vs. relationship set (2)
Previous figures displayed the relationship sets Each instance in the set relates individual participating entities – one from each participating entity type In ER diagrams, we represent the relationship type as follows: Diamond-shaped box is used to display a relationship type Connected to the participating entity types via straight lines

Refining the COMPANY database schema by introducing relationships
By examining the requirements, six relationship types are identified All are binary relationships( degree 2) Listed below with their participating entity types: WORKS_FOR (between EMPLOYEE, DEPARTMENT) MANAGES (also between EMPLOYEE, DEPARTMENT) CONTROLS (between DEPARTMENT, PROJECT) WORKS_ON (between EMPLOYEE, PROJECT) SUPERVISION (between EMPLOYEE (as subordinate), EMPLOYEE (as supervisor)) DEPENDENTS_OF (between EMPLOYEE, DEPENDENT)

ER Diagram For Company Database

Discussion on Relationship Types
In the refined design, some attributes from the initial entity types are refined into relationships: Manager of DEPARTMENT -> MANAGES Works_on of EMPLOYEE -> WORKS_ON Department of EMPLOYEE -> WORKS_FOR etc In general, more than one relationship type can exist between the same participating entity types MANAGES and WORKS_FOR are distinct relationship types between EMPLOYEE and DEPARTMENT With different meanings and different relationship instances.

Recursive Relationship Type
Is a relationship type with the same participating entity type in distinct roles Example: the SUPERVISION relationship EMPLOYEE participates twice in two distinct roles: supervisor (or boss) role supervisee (or subordinate) role Each relationship instance relates two distinct EMPLOYEE entities: One employee in supervisor role One employee in supervisee role

Example of relationships of different degrees
(a) Unary recursive relationships 22

Weak Entity Types An entity that does not have a key attribute
A weak entity must participate in an identifying relationship type with an owner or identifying entity type Entities are identified by the combination of: A partial key of the weak entity type The particular entity they are related to in the identifying entity type Example: A DEPENDENT entity is identified by the dependent’s first name, and the specific EMPLOYEE with whom the dependent is related Name of DEPENDENT is the partial key DEPENDENT is a weak entity type EMPLOYEE is its identifying entity type via the identifying relationship type DEPENDENT_OF

Weak Entity Type

Constraints on Relationships
Constraints on Relationship Types Cardinality Ratio (specifies maximum participation) (Also known as ratio constraints) One-to-one (1:1) One-to-many (1:N) or Many-to-one (N:1) Many-to-many (M:N) Existence Dependency Constraint (specifies minimum participation) (also called participation constraint) zero (optional participation, not existence-dependent) one or more (mandatory participation, existence-dependent)

Supervision Relationship

Manages Relationship

Recursive Relationship Type is: SUPERVISION (participation role names are shown)

Attributes of Relationship types
A relationship type can have attributes: For example, HoursPerWeek of WORKS_ON Its value for each relationship instance describes the number of hours per week that an EMPLOYEE works on a PROJECT. A value of HoursPerWeek depends on a particular (employee, project) combination Most relationship attributes are used with M:N relationships In 1:N relationships, they can be transferred to the entity type on the N-side of the relationship

Notation for Constraints on Relationships
Cardinality ratio (of a binary relationship): 1:1, 1:N, N:1, or M:N Shown by placing appropriate numbers on the relationship edges. Participation constraint (on each participating entity type): total (called existence dependency) or partial. Total shown by double line, partial by single line. NOTE: These are easy to specify for Binary Relationship Types.

Description Of Total And Partial Participant
1. MANAGES, a 1:1 relationship type between EMPLOYEE and DEPARTMENT. EMPLOYEE participation is partial. DEPARTMENT participation is not clear from the requirements. We question the users, who say that a department must have a manager at all times, which implies total participation. The attribute StartDate is assigned to this relationship type. 2. WORKSJOR, a I:N relationship type between DEPARTMENT and EMPLOYEE. Both participations are total. 3. CONTROLS, a I:N relationship type between DEPARTMENT and PROJECT. The participation of PROJECT is total, whereas that of DEPARTMENT is determined to be partial, after consultation with the users indicates that some departments may control no projects. 4. SUPERVISION, a I:N relationship type between EMPLOYEE (in the supervisor role) and EMPLOYEE (in the supervisee role). Both participations are determined to be partial, after the users indicate that not every employee is a supervisor and not every employee has a supervisor. 5. WORKS_ON, determined to be an M:N relationship type with attribute Hours, after the users indicate that a project can have several employees working on it. Both participations are determined to be total. 6. DEPENDENTS_OF, a l:N relationship type between EMPLOYEE and DEPENDENT, which is also the identifying relationship for the weak entity type DEPENDENT. The participation of EMPLOYEE is partial, whereas that of DEPENDENT is total.

Alternative (min, max) notation for relationship structural constraints:
Specified on each participation of an entity type E in a relationship type R Specifies that each entity e in E participates in at least min and at most max relationship instances in R Default(no constraint): min=0, max=n (signifying no limit) Must have minmax, min0, max 1 Derived from the knowledge of mini-world constraints Examples: A department has exactly one manager and an employee can manage at most one department. Specify (0,1) for participation of EMPLOYEE in MANAGES Specify (1,1) for participation of DEPARTMENT in MANAGES An employee can work for exactly one department but a department can have any number of employees. Specify (1,1) for participation of EMPLOYEE in WORKS_FOR Specify (0,n) for participation of DEPARTMENT in WORKS_FOR

Alternative diagrammatic notation
ER diagrams is one popular example for displaying database schemas Many other notations exist in the literature and in various database design and modeling tools Appendix A illustrates some of the alternative notations that have been used UML class diagrams is representative of another way of displaying ER concepts that is used in several commercial design tools

Summary of notation for ER diagrams

UML class diagrams Represent classes (similar to entity types) as large rounded boxes with three sections: Top section includes entity type (class) name Second section includes attributes Third section includes class operations (operations are not in basic ER model) Relationships (called associations) represented as lines connecting the classes Other UML terminology also differs from ER terminology Used in database design and object-oriented software design UML has many other types of diagrams for software design (see Chapter 12)

UML class diagram for COMPANY database schema

Other alternative diagrammatic notations

Relationships of Higher Degree
Relationship types of degree 2 are called binary Relationship types of degree 3 are called ternary and of degree n are called n-ary In general, an n-ary relationship is not equivalent to n binary relationships Constraints are harder to specify for higher-degree relationships (n > 2) than for binary relationships

Discussion of n-ary relationships (n > 2)
In general, 3 binary relationships can represent different information than a single ternary relationship (see Figure 3.17a and b on next slide) If needed, the binary and n-ary relationships can all be included in the schema design (see Figure 3.17a and b, where all relationships convey different meanings) In some cases, a ternary relationship can be represented as a weak entity if the data model allows a weak entity type to have multiple identifying relationships (and hence multiple owner entity types) (see Figure 3.17c)

Example of a ternary relationship

Discussion of n-ary relationships (n > 2)
If a particular binary relationship can be derived from a higher-degree relationship at all times, then it is redundant For example, the TAUGHT_DURING binary relationship in Figure 3.18 (see next slide) can be derived from the ternary relationship OFFERS (based on the meaning of the relationships)

Another example of a ternary relationship

Displaying constraints on higher-degree relationships
The (min, max) constraints can be displayed on the edges – however, they do not fully describe the constraints Displaying a 1, M, or N indicates additional constraints An M or N indicates no constraint A 1 indicates that an entity can participate in at most one relationship instance that has a particular combination of the other participating entities In general, both (min, max) and 1, M, or N are needed to describe fully the constraints

Data Modeling Tools A number of popular tools that cover conceptual modeling and mapping into relational schema design. Examples: ERWin, S- Designer (Enterprise Application Suite), ER- Studio, etc. POSITIVES: Serves as documentation of application requirements, easy user interface - mostly graphics editor support NEGATIVES: Most tools lack a proper distinct notation for relationships with relationship attributes Mostly represent a relational design in a diagrammatic form rather than a conceptual ER-based design (See Chapter 12 for details)

Some of the Currently Available Automated Database Design Tools
COMPANY TOOL FUNCTIONALITY Embarcadero Technologies ER Studio Database Modeling in ER and IDEF1X DB Artisan Database administration, space and security management Oracle Developer 2000/Designer 2000 Database modeling, application development Popkin Software System Architect 2001 Data modeling, object modeling, process modeling, structured analysis/design Platinum (Computer Associates) Enterprise Modeling Suite: Erwin, BPWin, Paradigm Plus Data, process, and business component modeling Persistence Inc. Pwertier Mapping from O-O to relational model Rational (IBM) Rational Rose UML Modeling & application generation in C++/JAVA Resolution Ltd. Xcase Conceptual modeling up to code maintenance Sybase Enterprise Application Suite Data modeling, business logic modeling Visio Visio Enterprise Data modeling, design/reengineering Visual Basic/C++

Extended Entity-Relationship (EER) Model (in next chapter)
The entity relationship model in its original form did not support the specialization and generalization abstractions Next chapter illustrates how the ER model can be extended with Type-subtype and set-subset relationships Specialization/Generalization Hierarchies Notation to display them in EER diagrams

Chapter Summary ER Model Concepts: Entities, attributes, relationships
Constraints in the ER model Using ER in step-by-step conceptual schema design for the COMPANY database ER Diagrams - Notation Alternative Notations – UML class diagrams, others